Essentials
of Materials Science and
Engineering
Second Edition
Donald R. Askeland
University of Missouri—Rolla, Emeritus
Pradeep P. Fulay
University of Pittsburgh
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To Suyash, Aarohee, and Jyotsna
— Pradeep P. Fulay
Contents
Preface xv
About the Authors xix
Chapter 1 Introduction to Materials Science and Engineering 1
Introduction 1
1-1
What is Materials Science and Engineering? 21-2
Classification of Materials 51-3
Functional Classification of Materials 91-4
Classification of Materials Based on Structure 111-5
Environmental and Other Effects 121-6
Materials Design and Selection 14SUMMARY 17
9GLOSSARY 18
9PROBLEMS 19
Chapter 2 Atomic Structure 21
Introduction 21
2-1
The Structure of Materials: Technological Relevance 222-2
The Structure of the Atom 232-3
The Electronic Structure of the Atom 282-4
The Periodic Table 302-5
Atomic Bonding 322-6
Binding Energy and Interatomic Spacing 40SUMMARY 44
9GLOSSARY 45
9PROBLEMS 48
Chapter 3 Atomic and Ionic Arrangements 51
Introduction 51
3-1
Short-Range Order versus Long-Range Order 523-2
Amorphous Materials: Principles and Technological Applications 543-3
Lattice, Unit Cells, Basis, and Crystal Structures 55vii
3-4
Allotropic or Polymorphic Transformations 633-5
Points, Directions, and Planes in the Unit Cell 643-6
Interstitial Sites 743-7
Crystal Structures of Ionic Materials 763-8
Covalent Structures 793-9
Diffraction Techniques for Crystal Structure Analysis 80SUMMARY 82
9GLOSSARY 83
9PROBLEMS 86
Chapter 4 Imperfections in the Atomic and Ionic Arrangements 90
Introduction 90
4-1
Point Defects 914-2
Other Point Defects 974-3
Dislocations 984-4
Significance of Dislocations 1054-5
Schmid’s Law 1054-6
Influence of Crystal Structure 1084-7
Surface Defects 1094-8
Importance of Defects 114SUMMARY 116
9GLOSSARY 117
9PROBLEMS 118
Chapter 5 Atom and Ion Movements in Materials 122
Introduction 122
5-1
Applications of Diffusion 1235-2
Stability of Atoms and Ions 1255-3
Mechanisms for Diffusion 1275-4
Activation Energy for Diffusion 1295-5
Rate of Diffusion (Fick’s First Law) 1305-6
Factors Affecting Diffusion 1335-7
Permeability of Polymers 1415-8
Composition Profile (Fick’s Second Law) 1425-9
Diffusion and Materials Processing 146SUMMARY 147
9GLOSSARY 148
9PROBLEMS 149
Chapter 6 Mechanical Properties: Fundamentals and Tensile, Hardness, and Impact Testing 153
Introduction 153
6-1
Technological Significance 1546-2
Terminology for Mechanical Properties 1556-3
The Tensile Test: Use of the Stress-Strain Diagram 1596-4
Properties Obtained from the Tensile Test 1636-5
True Stress and True Strain 1696-6
The Bend Test for Brittle Materials 1716-7
Hardness of Materials 1746-8
Strain Rate Effects and Impact Behavior 1766-9
Properties Obtained from the Impact Test 177SUMMARY 180
9GLOSSARY 181
9PROBLEMS 183
Chapter 7 Fracture Mechanics, Fatigue, and Creep Behavior 187
Introduction 187
7-1
Fracture Mechanics 1887-2
The Importance of Fracture Mechanics 1917-3
Microstructural Features of Fracture in Metallic Materials 1947-4
Microstructural Features of Fracture in Ceramics, Glasses, and Composites 1987-5
Weibull Statistics for Failure Strength Analysis 2007-6
Fatigue 2067-7
Results of the Fatigue Test 2097-8
Application of Fatigue Testing 2127-9
Creep, Stress Rupture, and Stress Corrosion 2157-10
Evaluation of Creep Behavior 217SUMMARY 220
9GLOSSARY 220
9PROBLEMS 222
Chapter 8 Strain Hardening and Annealing 225
Introduction 225
8-1
Relationship of Cold Working to the Stress-Strain Curve 2268-2
Strain-Hardening Mechanisms 2318-3
Properties versus Percent Cold Work 2328-4
Microstructure, Texture Strengthening, and Residual Stresses 2358-5
Characteristics of Cold Working 2398-6
The Three Stages of Annealing 2418-7
Control of Annealing 2448-8
Annealing and Materials Processing 2468-9
Hot Working 248SUMMARY 250
9GLOSSARY 250
9PROBLEMS 252
CONTENTS ix
Chapter 9 Principles and Applications of Solidification 257
Introduction 257
9-1
Technological Significance 2589-2
Nucleation 2599-3
Growth Mechanisms 2649-4
Cooling Curves 2699-5
Cast Structure 2719-6
Solidification Defects 2729-7
Casting Processes for Manufacturing Components 2749-8
Continuous Casting, Ingot Casting, and Single Crystal Growth 2769-9
Solidification of Polymers and Inorganic Glasses 2789-10
Joining of Metallic Materials 2799-11
Bulk Metallic Glasses (BMG) 280SUMMARY 282
9GLOSSARY 283
9PROBLEMS 286
Chapter 10 Solid Solutions and Phase Equilibrium 291
Introduction 291
10-1
Phases and the Phase Diagram 29210-2
Solubility and Solid Solutions 29610-3
Conditions for Unlimited Solid Solubility 29910-4
Solid-Solution Strengthening 30110-5
Isomorphous Phase Diagrams 30310-6
Relationship Between Properties and the Phase Diagram 31210-7
Solidification of a Solid-Solution Alloy 314SUMMARY 317
9GLOSSARY 318
9PROBLEMS 319
Chapter 11 Dispersion Strengthening and Eutectic Phase Diagrams 324
Introduction 324
11-1
Principles and Examples of Dispersion Strengthening 32511-2
Intermetallic Compounds 32611-3
Phase Diagrams Containing Three-Phase Reactions 32811-4
The Eutectic Phase Diagram 33111-5
Strength of Eutectic Alloys 34111-6
Eutectics and Materials Processing 34711-7
Nonequilibrium Freezing in the Eutectic System 349SUMMARY 350
9GLOSSARY 350
9PROBLEMS 352
Chapter 12 Dispersion Strengthening by Phase Transformations and Heat Treatment 357
Introduction 357
12-1
Nucleation and Growth in Solid-State Reactions 35812-2
Alloys Strengthened by Exceeding the Solubility Limit 36212-3
Age or Precipitation Hardening 36412-4
Applications of Age-Hardened Alloys 36412-5
Microstructural Evolution in Age or Precipitation Hardening 36512-6
Effects of Aging Temperature and Time 36712-7
Requirements for Age Hardening 36912-8
Use of Age-Hardenable Alloys at High Temperatures 36912-9
The Eutectoid Reaction 37012-10
Controlling the Eutectoid Reaction 37512-11
The Martensitic Reaction and Tempering 380SUMMARY 384
9GLOSSARY 385
9PROBLEMS 387
Chapter 13 Heat Treatment of Steels and Cast Irons 391
Introduction 391
13-1
Designations and Classification of Steels 39213-2
Simple Heat Treatments 39613-3
Isothermal Heat Treatments 39813-4
Quench and Temper Heat Treatments 40113-5
Effect of Alloying Elements 40613-6
Application of Hardenability 40913-7
Specialty Steels 41213-8
Surface Treatments 41513-9
Weldability of Steel 41713-10
Stainless Steels 41813-11
Cast Irons 422SUMMARY 428
9GLOSSARY 428
9PROBLEMS 431
Chapter 14 Nonferrous Alloys 436
Introduction 436
14-1
Aluminum Alloys 43814-2
Magnesium and Beryllium Alloys 44414-3
Copper Alloys 44714-4
Nickel and Cobalt Alloys 451CONTENTS xi
14-5
Titanium Alloys 45414-6
Refractory and Precious Metals 462SUMMARY 463
9GLOSSARY 463
9PROBLEMS 464
Chapter 15 Ceramic Materials 468
Introduction 468
15-1
Applications of Ceramics 46915-2
Properties of Ceramics 47115-3
Synthesis and Processing of Ceramic Powders 47215-4
Characteristics of Sintered Ceramics 47715-5
Inorganic Glasses 47915-6
Glass-Ceramics 48515-7
Processing and Applications of Clay Products 48715-8
Refractories 48815-9
Other Ceramic Materials 490SUMMARY 492
9GLOSSARY 493
9PROBLEMS 495
Chapter 16 Polymers 496
Introduction 496
16-1
Classification of Polymers 49716-2
Addition and Condensation Polymerization 50116-3
Degree of Polymerization 50416-4
Typical Thermoplastics 50616-5
Structure–Property Relationships in Thermoplastics 50916-6
Effect of Temperature on Thermoplastics 51216-7
Mechanical Properties of Thermoplastics 51816-8
Elastomers (Rubbers) 52316-9
Thermosetting Polymers 52816-10
Adhesives 53016-11
Polymer Processing and Recycling 531SUMMARY 537
9GLOSSARY 538
9PROBLEMS 540
Chapter 17 Composites: Teamwork and Synergy in Materials 543
Introduction 543
17-1
Dispersion-Strengthened Composites 54517-2
Particulate Composites 54717-3
Fiber-Reinforced Composites 55317-4
Characteristics of Fiber-Reinforced Composites 55717-5
Manufacturing Fibers and Composites 56417-6
Fiber-Reinforced Systems and Applications 56817-7
Laminar Composite Materials 57517-8
Examples and Applications of Laminar Composites 57717-9
Sandwich Structures 578SUMMARY 579
9GLOSSARY 580
9PROBLEMS 582
Appendix A: Selected Physical Properties of Some Elements 585 Appendix B: The Atomic and Ionic Radii of Selected Elements 587 Answers to Selected Problems 589
Index 592
CONTENTS xiii
Preface
This book,Essentials of Materials Science and Engineering Second Edition, is a direct result of the success of the Fifth Edition of The Science and Engineering of Materials, published in 2006. We received positive feedback on both the contents and the in- tegrated approach we used to develop materials science and engineering foundations by presenting the student with real-world applications and problems.
This positive feedback gave us the inspiration to develop Essentials of Materials Science and Engineering. The main objective of this book is to provide aconciseover- view of the principles of materials science and engineering for undergraduate students in varying engineering and science disciplines. This Essentials text contains the same in- tegrated approach as theFifth Edition, using real-world applications to present and then solve fundamental material science and engineering problems.
The contents of theEssentials of Materials Science and Engineeringbook have been carefully selected such that the reader can develop key ideas that are essential to a solid understanding of materials science and engineering. This book also contains several new examples of modern applications of advanced materials such as those used in informa- tion technology, energy technology, nanotechnology microelectromechanical systems (MEMS), and biomedical technology.
The concise approach used in this book will allow instructors to complete an in- troductory materials science and engineering course in one semester.
We feel that while reading and using this book, students will find materials science and engineering very interesting, and they will clearly see the relevance of what they are learning. We have presented many examples of modern applications of materials science and engineering that impact students’ lives. Our feeling is that if students recognize that many of today’s technological marvels depend on the availability of engineering materials they will be more motivated and remain interested in learning about how to apply the essentials of materials science and engineering.
Audience and Prerequisites
This book has been developed to cater to the needs of students from di¤erent engineer- ing disciplines and backgrounds other than materials science and engineering (e.g., me- chanical, industrial, manufacturing, chemical, civil, biomedical, and electrical engineer- ing). At the same time, a conscious e¤ort has been made so that the contents are very well suited for undergraduates majoring in materials science and engineering and closely related disciplines (e.g., metallurgy, ceramics, polymers, and engineering physics). In this sense, from a technical and educational perspective, the book has not been ‘‘watered down’’ in any way. The subjects presented in this text are a careful selection of topics based on our analysis of the needs and feedback from reviewers. Many of the topics xv
related to electronic, magnetic, thermal, and optical properties have not been included in this book to keep the page length down. For instructors and students who wish to develop these omitted concepts, we suggest using theFifth EditionofThe Science and Engineering of Materials.
This text is intended for engineering students who have completed courses in gen- eral physics, chemistry, physics, and calculus. Completion of a general introduction to Engineering or Engineering Technology will be helpful, but not necessary. The text does not presume that the students have had any engineering courses related to statics, dy- namics, or mechanics of materials.
Features
We have many unique features to this book.
Have You Ever Wondered? Questions Each chapter opens with a section entitled
‘‘Have You Ever Wondered?’’ These questions are designed to arouse the reader’s inter- est, put things in perspective, and form the framework for what the reader will learn in that chapter.
Examples Many real-world Examples have been integrated to accompany the chapter discussions. These Examples specifically cover design considerations, such as operating temperature, presence of corrosive material, economic considerations, recyclability, and environmental constraints. The examples also apply to theoretical material and numeric calculations to further reinforce the presentation.
Glossary All of the Glossary terms that appear in the chapter are set in boldface type the first time they appear within the text. This provides an easy reference to the defi- nitions provided in the end of each chapter Glossary.
Answers to Selected Problems The answers to the selected problems are provided at the end of the text to help the student work through the end-of-chapter problems.
Appendices and Endpapers Appendix A provides a listing of selected physical prop- erties of metals and Appendix B presents the atomic and ionic radii of selected ele- ments. The Endpapers include SI Conversion tables and Selected Physical Properties of elements.
Strategies for Teaching from the Book
Most of the material presented here can be covered in a typical one-semester course.
By selecting the appropriate topics, however, the instructor can emphasize the desired materials (i.e., metals, alloys, ceramics, polymers, composites, etc.), provide an over-
view of materials, concentrate on behavior, or focus on physical properties. In addition, the text provides the student with a useful reference for subsequent courses in manu- facturing, design, and materials selection. For students specializing in materials science and engineering, or closely related disciplines, sections related to synthesis and process- ing could be discussed in greater detail.
Supplements
Supplements for the instructor include:
9 The Instructor’s Solutions Manual that provides complete, worked-out solutions to selected text problems and additional text items.
9 Power Point slides of all figures from the textbook available from the book web- site at http://academic.cengage.com/engineering.
Acknowledgments
It takes a team of many people and a lot of hard work to create a quality textbook. We are indebted to all of the people who provided the assistance, encouragement, and con- structive criticism leading to the preparation of this book.
First, we wish to acknowledge the many instructors who have provided helpful feedback of bothThe Science and Engineering of MaterialsandEssentials of Materials Science and Engineering.
C. Maurice Balik, North Carolina State University
the late Deepak Bhat, University of Arkansass, Fayetteville Brian Cousins, University of Tasmania
Raymond Cutler, Ceramatec Inc.
Arthur F. Diaz, San Jose State University Phil Guichelaar, Western Michigan University Richard S. Harmer, University of Dayton Prashant N. Kumta, Carnegie Mellon University
Rafael Manory, Royal Melbourne Institute of Technology Sharon Nightingale, University of Wollongong, Australia Christopher K. Ober, Cornell University
David Poirier, University of Arizona
Ramurthy Prabhakaran, Old Dominion University Lew Rabenberg, The Unviersity of Texas at Austin Wayne Reitz, North Dakota State University John Schlup, Kansas State University
Robert L. Snyder, Georgia Institute of Technology J. Rasty, Texas Tech University
Acknowledgments xvii
Lisa Friis, University of Kansas
Blair London, California Polytechnic State University, San Luis Obispo Yu-Lin Shen, University of New Mexico
Stephen W. Sta¤ord, University of Texas at El Paso Rodney Trice, Purdue University
David S. Wilkinson, McMaster University Indranath Dutta, Naval Postgraduate School Richard B. Gri‰n, Texas A&M University
F. Scott Miller, Missouri University of Science and Technology Amod A. Ogale, Clemson University
Martin Pugh, Concordia University
Thanks most certainly to everyone at Cengage Learning for their encouragement, knowledge, and patience in seeing this text to fruition.
We wish to thank three people, in particular, for their diligent e¤orts: Many thanks to Chris Carson, our publisher, who set the tone for excellence and who provided the vision, expertise, and leadership to create such a quality product; to Hilda Gowans, our developmental editor and to Rose Kernan, our production editor, who worked long hours to improve our prose and produce this quality text from the first pages of manu- script to the final, bound product.
Pradeep Fulay would like specifically to thank his wife, Dr. Jyotsna Fulay and children, Aarohee and Suyash, for their patience, understanding, and encouragement.
Pradeep Fulay would also like to thank his parents Prabhakar and Pratibha Fulay for their support and encouragement. Thanks are also due to Professor S.H. Risbud, Uni- versity of California–Davis, for his advice and encouragement and to all of our col- leagues who provided many useful illustrations.
Donald R. Askeland
University of Missouri–Rolla, Emeritus Pradeep P. Fulay
University of Pittsburgh
About the Authors
Donald R. Askeland is a Distinguished Teaching Professor Emeritus of Metallurgical Engineering at the University of Missouri–Rolla. He received his degrees from the Thayer School of Engineering at Dartmouth College and the University of Michigan prior to joining the faculty at the University of Missouri–Rolla in 1970. Dr. Askeland taught a number of courses in materials and manufacturing engineering to students in a variety of engineering and science curricula. He received a number of awards for excel- lence in teaching and advising at UMR. He served as a Key Professor for the Foundry Educational Foundation and received several awards for his service to that organiza- tion. His teaching and research were directed primarily to metals casting and joining, in particular lost foam casting, and resulted in over 50 publications and a number of awards for service and best papers from the American Foundry Society.
xix
Dr. Pradeep Fulay has been a Professor of Materials Science and engineering in the Department of Mechanical Engineering and Materials Science for almost 19 years.
Currently, Dr. Fulay serves as the Program Director (PD) for the Electronic, Photonic Devices Technology Program (EPDT) at the National Science Foundation (NSF). He joined the University of Pittsburgh in 1989, was promoted to Associate Professor in 1994, and then to full professor in 1999. Dr. Fulay received a Ph.D. in Materials Science and Engineering from the University of Arizona (1989) and a B. Tech (1983) and M. Tech (1984) in Metallurgical Engineering from the Indian Institute of Technol- ogy Bombay (Mumbai) India.
He has authored close to 60 publications and has two U.S. patents issued. He has received the Alcoa Foundation and Ford Foundation research awards.
He has been an outstanding teacher and educator and was listed on the Faculty Honor Roll at the University of Pittsburgh (2001) for outstanding services and assis- tance. From 1992–1999, he was the William Kepler Whiteford Faculty Fellow at the University of Pittsburgh. From August to December 2002, Dr. Fulay was a visiting scientist at the Ford Scientific Research Laboratory in Dearborn, MI.
Dr. Fulay’s primary research areas are chemical synthesis and processing of ceramics, electronic ceramics and magnetic materials, development of smart materials and systems. He was the President of Ceramic Educational Council (2003–2004) and a Member of the Program Committee for the Electronics Division of the American ce- ramic society since 1996.
He has also served as an Associate Editor for theJournal of the American Ceramic Society (1994–2000). He has been the lead organizer for symposia on ceramics for sol-gel processing, wireless communications, and smart structures and sensors. In 2002, Dr. Fulay was elected as a Fellow of the American Ceramic Society. Dr. Fulay’s research has been supported by National Science Foundation (NSF) and other organizations.
1
Introduction to Materials Science and Engineering
Have You Ever Wondered?
9 Why do jewellers add copper to gold?
9 How sheet steel can be processed to produce a high-strength, lightweight, energy absorbing, malleable material used in the manufacture of car chassis?
9 Can we make flexible and lightweight electronic circuits using plastics?
9 What is a ‘‘smart material?’’
9 What is a superconductor?
In this chapter, we will introduce you to the field of materials science and engineering (MSE) using different real-world examples. We will then provide an introduction to the classification of materials. Materials science underlies most tech- nological advances. Understanding the basics of materials and their applications will not only
make you a better engineer, but will help you during the design process. In order to be a good designer, you must learn what materials will be appropriate to use in different applications. The most important aspect of materials is that they areenabling; materials make things happen. For example, in the history of civilization, materials 1
such as stone, iron, and bronze played a key role in mankind’s development. In today’s fast-paced world, the discovery of silicon single crystals and an understanding of their properties have en- abled the information age.
In this chapter and throughout the book, we will provide compelling examples of real-world applications of engineered materials. The diver- sity of applications and the unique uses of mate-
rials illustrate why an engineer needs to thor- oughly understand and know how to apply the principles of materials science and engineering.
In each chapter, we begin with a section entitled Have You Ever Wondered? These questions are designed to pique your curiosity, put things in perspective, and form a framework for what you will learn in that chapter.
1-1 What is Materials Science and Engineering?
Materials science and engineering (MSE) is an interdisciplinary field concerned with inventing new materials and improving previously known materials by developing a deeper understanding of the microstructure-composition-synthesis-processing relation- ships. The term composition means the chemical make-up of a material. The term structuremeans a description of the arrangement of atoms, as seen at di¤erent levels of detail. Materials scientists and engineers not only deal with the development of materi- als, but also with thesynthesisandprocessingof materials and manufacturing processes related to the production of components. The term ‘‘synthesis’’ refers to how materials are made from naturally occurring or man-made chemicals. The term ‘‘processing’’
means how materials are shaped into useful components. One of the most important functions of materials scientists and engineers is to establish the relationships between the properties of a material and its performance. Inmaterials science, the emphasis is on the underlying relationships between the synthesis and processing, structure, and prop- erties of materials. Inmaterials engineering, the focus is on how to translate or trans- form materials into a useful device or structure.
One of the most fascinating aspects of materials science involves the investigation into the structure of a material. The structure of materials has a profound influence on many properties of materials, even if the overall composition does not change! For ex- ample, if you take a pure copper wire and bend it repeatedly, the wire not only becomes harder but also becomes increasingly brittle! Eventually, the pure copper wire becomes so hard and brittle that it will break rather easily. The electrical resistivity of wire will also increase as we bend it repeatedly. In this simple example, note that we did not change the material’s composition (i.e., its chemical make up). The changes in the ma- terial’s properties are often due to a change in its internal structure. If you examine the wire after bending using an optical microscope, it will look the same as before (other than the bends, of course). However, its structure has been changed at a very small or microscopic scale. The structure at this microscopic scale is known asmicrostructure. If we can understand what has changed at a micrometer level, we can begin to discover ways to control the material’s properties.
Let’s put the materials science and engineering tetrahedron in perspective by ex- amining a sample product–ceramic superconductors invented in 1986 (Figure 1-1). You may be aware that ceramic materials usually do not conduct electricity. Scientists found, serendipitously, that certain ceramic compounds based on yttrium barium copper oxides (known as YBCO) can actually carry electrical current without any resistance under certain conditions. Based on what was known then about metallic supercon- ductors and the electrical properties of ceramics, superconducting behavior in ceramics was not considered as a strong possibility. Thus, the first step in this case was thedis- covery of superconducting behavior in ceramic materials. These materials were dis- covered through some experimental research. A limitation of these materials is that they can superconduct only at low temperatures (<150 K).
The next step was to determine how to make these materials better. By ‘‘better’’ we mean: How can we retain superconducting behavior in these materials at higher tem- peratures, or how can we transport a large amount of current over a long distance? This involves materials processing and careful structure-property studies. Materials scientists wanted to know how the composition and microstructure a¤ect the superconducting Figure 1-1 Application of the tetrahedron of materials science and engineering to ceramic superconductors. Note that the microstructure-synthesis and processing-composition are all interconnected and affect the performance-to-cost ratio.
1-1 What is Materials Science and Engineering? 3
behavior. They also want to know if there are other compounds that exhibited super- conductivity. Through experimentation, the scientists developed controlledsynthesisof ultrafine powders or thin films that are used to create useful devices.
An example of approaching this from amaterials engineeringperspective will be to find a way to make long wires for power transmission. In applications, we ultimately want to know if we can make reliable and reproducible long lengths of superconducting wires that are superior to the current copper and aluminum wires. Can we produce such wires in a cost-e¤ective way?
The next challenge was to make long lengths of ceramic superconductor wires.
Ceramic superconductors are brittle, so making long lengths of wires was di‰cult.
Thus,materials processing techniques had to be developed to create these wires. One successful way of creating these superconducting wires was to fill hollow silver tubes with powders of superconductor ceramic and then draw wires.
Although the discovery of ceramic superconductors did cause a lot of excitement, the path toward translating that discovery into useful products has been met by many challenges related to the synthesis and processing of these materials.
Sometimes, discoveries of new materials, phenomena, or devices are heralded as revolutionary. Today, as we look back, the 1948 discovery of the silicon-based transistor used in computer chips is considered revolutionary. On the other hand, materials that have evolved over a period of time can be just as important. These materials are con- sidered asevolutionary. Many alloys based on iron, copper, and the like are examples of evolutionary materials. Of course, it is important to recognize that what are considered as evolutionary materials now, did create revolutionary advances many years back. It is not uncommon for materials or phenomena to be discovered first and then for many years to go by before commercial products or processes appear in the marketplace. The transition from the development of novel materials or processes to useful commercial or industrial applications can be slow and di‰cult.
Let’s examine another example using the materials science and engineering tetra- hedron. Let’s look at ‘‘sheet steels’’ used in the manufacture of car chassis. Steels, as you may know, have been used in manufacturing for more than a hundred years.
Earlier steels probably existed in a crude form during the Iron Age, thousands of years ago. In the manufacture of automobile chassis, a material is needed that possesses ex- tremely high strength but is easily formed into aerodynamic contours. Another consid- eration is fuel-e‰ciency, so the sheet steel must also be thin and lightweight. The sheet steels should also be able to absorb significant amounts of energy in the event of a crash, thereby increasing vehicle safety. These are somewhat contradictory require- ments.
Thus, in this case, materials scientists are concerned with the sheet steel’s
9 composition;
9 strength;
9 density;
9 energy absorption properties; and
9 ductility (formability).
Materials scientists would examine steel at a microscopic level to determine if its properties can be altered to meet all of these requirements. They also would have to process this material into a car chassis in a cost-e¤ective way. Will the shaping process itself a¤ect the mechanical properties of the steel? What kind of coatings can be devel- oped to make the steel corrosion-resistant? We also need to know if these steels could be welded easily. From this discussion, you can see that many issues need to be considered during the design and materials selection for any product.
1-2 Classification of Materials
There are di¤erent ways of classifying materials. One way is to describe five groups (Table 1-1):
1. metalsandalloys;
2. ceramics,glasses, andglass-ceramics;
3. polymers (plastics);
4. semiconductors; and 5. composite materials.
Materials in each of these groups possess di¤erent structures and properties. The di¤erences in strength, which are compared in Figure 1-2, illustrate the wide range of properties engineers can select from. Since metallic materials are extensively used for
TABLE 1-19Representative examples, applications, and properties for each category of materials Examples of Applications Properties
Metals and Alloys
Copper Electrical conductor wire High electrical conductivity, good formability
Gray cast iron Automobile engine blocks Castable, machinable, vibration- damping
Alloy steels Wrenches, automobile chassis Significantly strengthened by heat treatment
Ceramics and Glasses
SiO2-Na2O-CaO Window glass or soda-lime glass Optically transparent, thermally insulating
Al2O3, MgO, SiO2 Refractories (i.e., heat-resistant lining of furnaces) for containing molten metal
Thermally insulating, withstand high temperatures, relatively inert to molten metal Barium titanate Capacitors for microelectronics High ability to store charge Silica Optical fibers for information
technology
Refractive index, low optical losses
Polymers
Polyethylene Food packaging Easily formed into thin, flexible,
airtight film
Epoxy Encapsulation of integrated circuits Electrically insulating and moisture-resistant Phenolics Adhesives for joining plies in plywood Strong, moisture resistant Semiconductors
Silicon (Si) Transistors and integrated circuits Unique electrical behavior
GaAs Optoelectronic systems Converts electrical signals to
light, lasers, laser diodes, etc.
Composites
Graphite-epoxy Aircraft components High strength-to-weight ratio Tungsten carbide-cobalt
(WC-Co)
Carbide cutting tools for machining High hardness, yet good shock resistance
Titanium-clad steel Reactor vessels Low cost and high strength of steel, with the corrosion resistance of titanium
1-2 Classification of Materials 5
load-bearing applications, their mechanical properties are of great practical interest. We briefly introduce these here. The term ‘‘stress’’ refers to load or force per unit area.
‘‘Strain’’ refers to elongation or change in dimension divided by original dimension.
Application of ‘‘stress’’ causes ‘‘strain.’’ If the strain goes away after the load or applied stress is removed, the strain is said to be ‘‘elastic.’’ If the strain remains after the stress is removed, the strain is said to be ‘‘plastic.’’ When the deformation is elastic, stress and strain are linearly related, the slope of the stress-strain diagram is known as the elastic or Young’s modulus. A level of stress needed to initiate plastic deformation is known as
‘‘yield strength.’’ The maximum percent deformation we can get is a measure of the ductility of a metallic material. These concepts are discussed further in Chapter 6.
Metals and Alloys These include steels, aluminum, magnesium, zinc, cast iron, titanium, copper, and nickel. In general, metals have good electrical and thermal con- ductivity. Metals and alloys have relatively high strength, high sti¤ness, ductility or formability, and shock resistance. They are particularly useful for structural or load- bearing applications. Although pure metals are occasionally used, combinations of metals called alloys provide improvement in a particular desirable property or permit better combinations of properties. The cross section of a jet engine shown in Figure 1-3 illustrates the use of metallic materials for a number of critical applications.
Ceramics Ceramics can be defined as inorganic crystalline materials. Ceramics are probably the most ‘‘natural’’ materials. Beach sand and rocks are examples of naturally occurring ceramics. Advanced ceramics are materials made by refining naturally occur- ring ceramics and other special processes. Advanced ceramics are used in substrates that house computer chips, sensors and actuators, capacitors, spark plugs, inductors, and electrical insulation. Some ceramics are used as thermal-barrier coatings to protect metallic substrates in turbine engines. Ceramics are also used in such consumer products as paints, plastics, tires, and for industrial applications such as the tiles for the space Figure 1-2 Representative strengths of various categories of materials. The strength of
ceramics is under a compressive stress.
shuttle, a catalyst support, and oxygen sensors used in cars. Traditional ceramics are used to make bricks, tableware, sanitaryware, refractories (heat-resistant material), and abrasives. In general, due to the presence of porosity (small holes), ceramics tend to be brittle. Ceramics must also be heated to very high temperatures before they can melt.
Ceramics are strong and hard, but also very brittle. We normally prepare fine powders of ceramics and convert these into di¤erent shapes. New processing techniques make ceramics su‰ciently resistant to fracture that they can be used in load-bearing applica- tions, such as impellers in turbine engines (Figure 1-4). Ceramics have exceptional Figure 1-3 A section through a jet engine. The forward compression section operates at low to medium temperatures, and titanium parts are often used. The rear combustion section operates at high temperatures and nickel-based superalloys are required. The outside shell experiences low temperatures, and aluminum and composites are satisfactory. (Courtesy of GE Aircraft Engines.)
Figure 1-4 A variety of complex ceramic components, including impellers and blades, which allow turbine engines to operate more efficiently at higher temperatures. (Courtesy of Certech, Inc.)
1-2 Classification of Materials 7
strength under compression (Figure 1-2). Can you believe that the weight of an entire fire truck can be supported using four ceramic co¤ee cups?
Glasses and Glass-Ceramics Glass is an amorphous material, often, but not always, derived from molten silica. The term ‘‘amorphous’’ refers to materials that do not have a regular, periodic arrangement of atoms. Amorphous materials will be discussed in detail in Chapter 3. The fiber optics industry is founded on optical fibers made by using high-purity silica glass. Glasses are also used in houses, cars, computer and television screens, and hundreds of other applications. Glasses can be thermally treated (tem- pered) to make them stronger. Forming glasses and nucleating (creating) small crystals within them by a special thermal process creates materials that are known as glass- ceramics. ZerodurTMis an example of a glass-ceramic material that is used to make the mirror substrates for large telescopes (e.g., the Chandra and Hubble telescopes). Glasses and glass-ceramics are usually processed by melting and casting.
Polymers Polymers are typically organic materials produced using a process known as polymerization. Polymeric materials include rubber (elastomers) and many types of ad- hesives. Many polymers have very good electrical resistivity. They can also provide good thermal insulation. Although they have lower strength, polymers have a very good strength-to-weight ratio. They are typically not suitable for use at high temperatures.
Many polymers have very good resistance to corrosive chemicals. Polymers have thou- sands of applications ranging from bulletproof vests, compact disks (CDs), ropes, and liquid crystal displays (LCDs) to clothes and co¤ee cups.Thermoplastic polymers, in which the long molecular chains are not rigidly connected, have good ductility and formability;thermosettingpolymers are stronger but more brittle because the molecular chains are tightly linked (Figure 1-5). Polymers are used in many applications, including electronic devices. Thermoplastics are made by shaping their molten form. Thermosets are typically cast into molds. The termplasticsis used to describe polymeric materials containing additives that enhance their properties.
Semiconductors Silicon, germanium, and gallium arsenide-based semiconductors are part of a broader class of materials known as electronic materials. The electrical con- ductivity of semiconducting materials is between that of ceramic insulators and metallic conductors.Semiconductorshave enabled the information age. In semiconductors, the
Figure 1-5 Polymerization occurs when small molecules, represented by the circles, combine to produce larger molecules, or polymers. The polymer molecules can have a structure that consists of many chains that are entangled but not connected (thermoplastics) or can form three-dimensional networks in which chains are cross-linked (thermosets).
level of conductivity is controlled to enable their use in electronic devices such as tran- sistors, diodes, etc., that are used to build integrated circuits. In many applications, we need large single crystals of semiconductors. These are grown from molten materials.
Often, thin films of semiconducting materials are also made using specialized processes.
Composite Materials The main idea in developingcompositesis to blend the properties of di¤erent materials. The composites are formed from two or more materials, produc- ing properties not found in any single material. Concrete, plywood, and fiberglass are examples of composite materials. Fiberglass is made by dispersing glass fibers in a polymer matrix. The glass fibers make the polymer matrix sti¤er, without significantly increasing its density. With composites we can produce lightweight, strong, ductile, high temperature-resistant materials or we can produce hard, yet shock-resistant, cutting tools that would otherwise shatter. Advanced aircraft and aerospace vehicles rely heav- ily on composites such as carbon-fiber-reinforced polymers. Sports equipment such as bicycles, golf clubs, tennis rackets, and the like also make use of di¤erent kinds of composite materials that are light and sti¤.
1-3 Functional Classification of Materials
We can classify materials based on whether the most important function they perform is mechanical (structural), biological, electrical, magnetic, or optical. This classification of materials is shown in Figure 1-6. Some examples of each category are shown. These categories can be broken down further into subcategories.
Aerospace Light materials such as wood and an aluminum alloy (that accidentally strengthened the alloy used for making the engine even more by picking up copper from the mold used for casting) were used in the Wright brothers’ historic flight. Aluminum alloys, plastics, silica for space shuttle tiles, carbon-carbon composites, and many other materials belong to this category.
Biomedical Our bones and teeth are made, in part, from a naturally formed ceramic known as hydroxyapatite. A number of artificial organs, bone replacement parts, car- diovascular stents, orthodontic braces, and other components are made using di¤erent plastics, titanium alloys, and nonmagnetic stainless steels. Ultrasonic imaging systems make use of ceramics known as PZT (lead zirconium titanate). Magnets used for magnetic resonance imaging make use of metallic niobium tin-based superconductors.
Electronic Materials As mentioned before, semiconductors, such as those made from silicon, are used to make integrated circuits for computer chips. Barium titanate (BaTiO3), tantalum oxide (Ta2O5), and many other dielectric materials are used to make ceramic capacitors and other devices. Superconductors are used in making powerful magnets. Copper, aluminum, and other metals are used as conductors in power transmission and in microelectronics.
Energy Technology and Environmental Technology The nuclear industry uses materi- als such as uranium dioxide and plutonium as fuel. Numerous other materials, such as glasses and stainless steels, are used in handling nuclear materials and managing radio- active waste. New technologies related to batteries and fuel cells make use of many ceramic materials such as zirconia (ZrO2) and polymers. The battery technology has 1-3 Functional Classification of Materials 9
gained significant importance owing to the need for many electronic devices that require longer lasting and portable power. Fuel cells are also being used in some cars. The oil and petroleum industry widely uses zeolites, alumina, and other materials as catalyst substrates. They use Pt, Pt/Rh and many other metals as catalysts. Many membrane technologies for purification of liquids and gases make use of ceramics and plastics.
Solar power is generated using materials such as crystalline Si and amorphous silicon (a:Si:H).
Magnetic Materials Computer hard disks and audio and video cassettes make use of many ceramic, metallic, and polymeric materials. For example, particles of a special Figure 1-6 Functional classification of materials. Notice that metals, plastics, and ceramics occur in different categories. A limited number of examples in each category are provided.
form of iron oxide, known as gamma iron oxide (g-Fe2O3) are deposited on a polymer substrate to make audio cassettes. High-purity iron particles are used for making video- tapes. Computer hard disks are made using alloys based on cobalt-platinum-tantalum- chromium (Co-Pt-Ta-Cr) alloys. Many magnetic ferrites are used to make inductors and components for wireless communications. Steels based on iron and silicon are used to make transformer cores.
Photonic or Optical Materials Silica is used widely for making optical fibers. Almost ten million kilometers of optical fiber have been installed around the world. Optical materials are used for making semiconductor detectors and lasers used in fiber optic communications systems and other applications. Similarly, alumina (Al2O3) and yttrium aluminum garnets (YAG) are used for making lasers. Amorphous silicon is used to make solar cells and photovoltaic modules. Polymers are used to make liquid crystal displays (LCDs).
Smart Materials Asmart materialcan sense and respond to an external stimulus such as a change in temperature, the application of a stress, or a change in humidity or chemical environment. Usually a smart-material-based system consists of sensors and actuators that read changes and initiate an action. An example of a passively smart material is lead zirconium titanate (PZT) and shape-memory alloys. When properly processed, PZTcan be subjected to a stress and a voltage is generated. This e¤ect is used to make such devices as spark generators for gas grills and sensors that can detect underwater objects such as fish and submarines. Other examples of smart materials include magnetorheological or MR fluids. These are magnetic paints that respond to magnetic fields and are being used in suspension systems of automobiles. Other exam- ples of smart materials and systems are photochromic glasses and automatic dimming mirrors based on electrochromic materials.
Structural Materials These materials are designed for carrying some type of stress.
Steels, concrete, and composites are used to make buildings and bridges. Steels, glasses, plastics, and composites are also used widely to make automotives. Often in these applications, combinations of strength, sti¤ness, and toughness are needed under dif- ferent conditions of temperature and loading.
1-4 Classification of Materials Based on Structure
As mentioned before, the term ‘‘structure’’ means the arrangement of a material’s atoms; the structure at a microscopic scale is known as ‘‘microstructure.’’ We can view these arrangements at di¤erent scales, ranging from a few angstrom units to a millime- ter. We will learn in Chapter 3 that some materials may becrystalline(where the mate- rial’s atoms are arranged in a periodic fashion) or they may be amorphous (where the material’s atoms do not have a long-range order). Some crystalline materials may be in the form of one crystal and are known as single crystals. Others consist of many crystals or grainsand are known as polycrystalline. The characteristics of crystals or grains (size, shape, etc.) and that of the regions between them, known as the grain boundaries, also a¤ect the properties of materials. We will further discuss these concepts in later chapters. A micrograph of a stainless steel sample (showing grains and grain boundaries) is shown in Figure 1-7. For this sample, each grain reflects the light di¤er- ently and this produces a contrast between the grains.
1-4 Classification of Materials Based on Structure 11
1-5 Environmental and Other Effects
The structure-property relationships in materials fabricated into components are often influenced by the surroundings to which the material is subjected during use. This can include exposure to high or low temperatures, cyclical stresses, sudden impact, corro- sion or oxidation. These e¤ects must be accounted for in design to ensure that compo- nents do not fail unexpectedly.
Temperature Changes in temperature dramatically alter the properties of materials (Figure 1-8). Metals and alloys that have been strengthened by certain heat treatments or forming techniques will lose their strength when heated. A tragic reminder of this is the collapse of the steel beams used in the World Trade Center towers on September 11, 2001.
High temperatures change the structure of ceramics and cause polymers to melt or char. Very low temperatures, at the other extreme, may cause a metal or polymer to fail in a brittle manner, even though the applied loads are low. This low temperature em-
Figure 1-7
Micrograph of stainless steel showing grains and grain boundaries.
(Courtesy Dr. Hua and Dr. DeArdo—
University of Pittsburgh.)
Figure 1-8
Increasing temperature normally reduces the strength of a material.
Polymers are suitable only at low temperatures. Some composites, such as carbon-carbon composites, special alloys, and ceramics, have excellent properties at high temperatures.
brittlement was a factor that caused theTitanicto fracture and sink. Similarly, the 1986 Challenger accident, in part, was due to embrittlement of rubber O-rings. The reasons why some polymers and metallic materials become brittle are di¤erent. We will discuss these concepts in later chapters.
The design of materials with improved resistance to temperature extremes is essen- tial in many technologies related to aerospace. As faster speeds are attained, more heating of the vehicle skin occurs because of friction with the air. At the same time, engines operate more e‰ciently at higher temperatures. So, in order to achieve higher speed and better fuel economy, new materials have gradually increased allowable skin and engine temperatures. But materials engineers are continually faced with new chal- lenges. TheX-33 andVenturestarare examples of advanced reusable vehicles intended to carry passengers into space using a single stage of rocket engines. Figure 1-9 shows a schematic of the X-33prototype. The development of even more exotic materials and processing techniques is necessary in order to tolerate the high temperatures that will be encountered.
Corrosion Most of the time, failure of materials occurs as a result of corrosion and some form of tensile overload. Most metals and polymers react with oxygen or other gases, particularly at elevated temperatures. Metals and ceramics may disintegrate and polymers and nonoxide ceramics may oxidize. Materials are also attacked by corrosive liquids, leading to premature failure. The engineer faces the challenge of selecting ma- terials or coatings that prevent these reactions and permit operation in extreme envi- ronments. In space applications, we may have to consider the e¤ects of the presence of radiation, the presence of atomic oxygen, and the impact from debris.
Figure 1-9 Schematic of aX-33plane prototype. Notice the use of different materials for different parts. This type of vehicle will test several components for theVenturestar(From
‘‘A Simpler Ride into Space,’’ by T.K. Mattingly, October, 1997,Scientific American,p. 125.
Copyright>1997 Slim Films.)
1-5 Environmental and Other Effects 13
Fatigue In many applications, components must be designed such that the load on the material may not be enough to cause permanent deformation. However, when we do load and unload the material thousands of times, small cracks may begin to develop and materials fail as these cracks grow. This is known asfatigue failure. In designing load-bearing components, the possibility of fatigue must be accounted for.
Strain Rate You may be aware of the fact that Silly Putty8, a silicone- (not silicon-) based plastic, can be stretched significantly if we pull it slowly (small rate of strain). If you pull it fast (higher rate of strain) it snaps. A similar behavior can occur with many metallic materials. Thus, in many applications, the level and rate of strain have to be considered.
In many cases, the e¤ects of temperature, fatigue, stress, and corrosion may be interrelated, and other outside e¤ects could a¤ect the material’s performance.
1-6 Materials Design and Selection
When a material is designed for a given application, a number of factors must be considered. The material must possess the desired physical and mechanical properties.
It must be capable of being processed or manufactured into the desired shape, and must provide an economical solution to the design problem. Satisfying these requirements in a manner that protects the environment—perhaps by encouraging recycling of the materials—is also essential. In meeting these design requirements, the engineer may have to make a number of tradeo¤s in order to produce a serviceable, yet marketable, product.
As an example, material cost is normally calculated on a cost-per-pound basis. We must consider thedensity of the material, or its weight-per-unit volume, in our design and selection (Table 1-2). Aluminum may cost more per pound than steel, but it is only one-third the weight of steel. Although parts made from aluminum may have to be thicker, the aluminum part may be less expensive than the one made from steel because of the weight di¤erence.
TABLE 1-29Strength-to-weight ratios of various materials
Strength Density Strength-to-weight
Material (lb/in.2) (lb/in.3) ratio (in.)
Polyethylene 1,000 0.030 0.03106
Pure aluminum 6,500 0.098 0.07106
Al2O3 30,000 0.114 0.26106
Epoxy 15,000 0.050 0.30106
Heat-treated alloy steel 240,000 0.280 0.86106
Heat-treated aluminum alloy 86,000 0.098 0.88106
Carbon-carbon composite 60,000 0.065 0.92106
Heat-treated titanium alloy 170,000 0.160 1.06106
Kevlar-epoxy composite 65,000 0.050 1.30106
Carbon-epoxy composite 80,000 0.050 1.60106
In some instances, particularly in aerospace applications, the weight issue is critical, since additional vehicle weight increases fuel consumption and reduces range. By using materials that are lightweight but very strong, aerospace or automobile vehicles can be designed to improve fuel e‰ciency. Many advanced aerospace vehicles use composite materials instead of aluminum alloys. These composites, such as carbon-epoxy, are more expensive than the traditional aluminum alloys; however, the fuel savings yielded by the higher strength-to-weight ratio of the composite (Table 1-2) may o¤set the higher initial cost of the aircraft. The body of one of the latest Boeing aircrafts known as the Dreamliner is made almost entirely from carbon-carbon composite materials. There are literally thousands of applications in which similar considerations apply. Usually the selection of materials involves trade-o¤s between many properties.
By this point of our discussion, we hope that you can appreciate that the properties of materials depend not only on composition, but also on how the materials are made (synthesis and processing) and, most importantly, their internal structure. This is why it is not a good idea for an engineer to simply refer to a handbook and select a material for a given application. The handbooks may be a good starting point. A good engineer will consider: the e¤ects of how the material is made, what exactly is the composition of the candidate material for the application being considered, any processing that may have to be done for shaping the material or fabricating a component, the structure of the material after processing into a component or device, the environment in which the material will be used, and the cost-to-performance ratio. The knowledge of principles of materials science and engineering will empower you with the fundamental concepts.
These will allow you to make technically sound decisions in designing with engineered materials.
EXAMPLE 1-1
Materials for a Bicycle FrameBicycle frames are made using steel, aluminum alloys, titanium alloys contain- ing aluminum and vanadium, and carbon-fiber composites (Figure 1-10). (a) If a steel-frame bicycle weighs 30 pounds, what will be the weight of the frame assuming we use aluminum, titanium, and a carbon-fiber composite to make the frame in such a way that the volume of frame (the diameter of the tubes) is constant? (b) What other considerations can come into play in designing bicy- cle frames?
Figure 1-10
Bicycle frames need to be lightweight, stiff, and corrosion resistant (for Example 1-1). (Courtesy of Chris harve/StockXpert.)
1-6 Materials Design and Selection 15
Note: The densities of steel, aluminum alloy, titanium alloy, and carbon- fiber composite can be assumed to be 7.8, 2.7, 4.5, and 1.85 g/cm3.
SOLUTION
(a) The weight of the bicycle frame made from steel is stated to be 30 pounds.
The volume of this frame will be
Vframe¼ ð30454 g=lbÞ=ð7:8Þ ¼1746 cm3 For aluminum frame the weight will be
Wal¼ ð1746 cm3Þ ð2:7 g=cm3Þ ð1 lb=454Þgrams¼10:38 lbs Another and simpler way to arrive at this answer is to take the ratio of densities, since the volume is assumed constant.
The weight of the aluminum alloy frame
Walloy¼ ðdensity of aluminum alloy=density of steelÞ ðwt:of the steel frameÞ
¼ ð2:7=7:8Þ 30 lb¼10:38 lb
Thus, the aluminum frame weighs roughly one-third of the steel frame.
Similarly, the weight of titanium frame will be
WTi¼ ðdensity of titanium alloy=density of steelÞ ðwt:of the steel frameÞ
¼ ð4:5=7:8Þ 30 lb¼17:3 lb
Finally, the weight of the frame made using carbon-fiber composite will be Wcf ¼ ðdensity of carbon fiber composite=density of steelÞ
ðwt:of the steel frameÞ
¼ ð1:85=7:8Þ 30 lb¼7:1 lb
As can be seen, substantial reduction in weight is possible using materials other than steel.
(b) One of the other factors that comes into play is the sti¤ness of the structure.
This is related to the elastic modulus of the material (Chapter 2). For ex- ample, for the same tube dimensions, an aluminum tube will be not as sti¤
as steel. This will make the aluminum frame bicycle ride ‘‘soft.’’ This e¤ect can be compensated for by making the aluminum tubes larger in diameter and the walls of the tubes thicker. Some other factors to consider are the toughness of each of the materials. For example, even though a carbon- fiber frame is very light, it is relatively brittle. Additional considerations would be the ability to weld or join the frame to other parts of the bicycle, corrosion resistance, and of course, cost.
EXAMPLE 1-2
Ceramic-Carbon-Fiber Brakes for CarsCar breaks are typically made using cast iron and weigh about 20 pounds.
What other materials can be used to make brakes that would last long and weigh less?
SOLUTION
The brakes could be made using other lower density materials, such as alumi- num or titanium. Cost and wear resistance are clearly important. Titanium alloys will be very expensive, and both titanium and aluminum will wear out more easily.
We could make the brakes out of ceramics, such as alumina (Al2O3) or silicon carbide (SiC), since both have densities lower than cast iron. However, ceramics are too brittle, and even though they have very good resistance, they will fracture easily.
We can use a material that is a composite of carbon fibers and ceramics, such as SiC. This composite material will provide the lightweight and wear- resistance necessary, so that the brakes do not have to be replaced often. Some companies are already producing such ceramic-carbon-fiber brakes.
SUMMARY V
The properties of engineered materials depend upon their composition, structure, synthesis, and processing. An important performance index for materials or devices is their cost-to-performance ratio.V
The structure at a microscopic level is known as the microstructure (length scale 10 nm to 1000 nm).V
Many properties of materials depend strongly on the structure, even if the com- position of the material remains the same. This is why the structure-property or microstructure-property relationships in materials are extremely important.V
Materials are often classified as metals and alloys, ceramics, glasses, and glass ceramics, composites, polymers, and semiconductors.V
Metals and alloys have good strength, good ductility, and good formability. Pure metals have good electrical and thermal conductivity. Metals and alloys play an indispensable role in many applications such as automotives, buildings, bridges, aerospace, and the like.V
Ceramics are inorganic, crystalline materials. They are strong, serve as good elec- trical and thermal insulators, are often resistant to damage by high temperatures and corrosive environments, but are mechanically brittle. Modern ceramics form the underpinnings of many of the microelectronic and photonic technologies.V
Polymers have relatively low strength; however, the strength-to-weight ratio is very favorable. Polymers are not suitable for use at high temperatures. They have very good corrosion resistance, and—like ceramics—provide good electrical and thermal insulation. Polymers may be either ductile or brittle, depending on struc- ture, temperature, and the strain rate.V
Materials can also be classified as crystalline or amorphous. Crystalline materials may be single crystal or polycrystalline.Summary 17
